Tunable liquid crystal (LC) optical devices, such as lenses, beam steering devices and shutters are known in the art. While some tunable LC lenses operate with a uniform electric or magnetic control field, most use a spatially modulated field. In the case of electric fields, there are a few prior art techniques used to spatially modulate the electric field. Spatially inhomogeneous dielectric layers have been used to attenuate the electric field to have a desired spatial profile. Electrodes have been spherically shaped to provide a desired spatial profile to the electric field. Another approach to spatially modulating the electric field is to use a planar electrode whose impedance properties are such that the voltage drop over the electrode as AC drive current is fed to the electrode leads to a spatially modulated electric field.
As shown in FIG. 1, one type of conventional LC cell is built by sandwiching the liquid crystal 102 between two substrates 104,106, each of which is first coated by a transparent electrode 108, 110, which may be a layer of material such as indium tin oxide (ITO), and then coated by polymer layers 112 (typically polyimide) which are rubbed in a predetermined direction to align LC molecules in a ground state, namely in the absence of the controlling electric field. The application of voltage to two ITOs creates a uniform electric field and correspondingly uniform LC reorientation (and correspondingly uniform refractive index distribution). In such a device, the index of refraction is different in a direction lengthwise with respect to the molecules than transverse to the molecules.
FIG. 2 illustrates a prior art LC cell configuration, in which a hole patterned electrode ring 204 of low resistivity surrounding a disk-shaped zone 205 of high resistivity material is used to generate an electric field gradient thanks to its strong “resonant” attenuation. This geometry has the advantages of being very thin (which is a key requirement, e.g., in cell phone applications) and of using only two electrodes (and thus one voltage for control). Unfortunately, it is difficult to produce the required thickness of high resistivity material with high optical transparency, as well as an LC cell with good uniformity, and the manufacturing process typically has a low yield. Different lenses will have slightly different electrode resistances and this, coupled with the fact that modal control is also very dependent on the precise cell thickness, means that each individual lens needs to be calibrated separately. Also, the minimum diameter of a modal lens is limited to about 2 mm—below this size the required resistivity of the ITO layer exceeds some 10MΩ/sq. Finally, such (so called “modal control”) lenses must always be either positive or negative. It is not possible to switch between a diverging and converging lens.
FIG. 3 illustrates another prior art LC cell configuration with electric field gradient generation, using three distinct electrodes 304, 305, 307 (two of them in the inter-hole pattern formed on the same plane), two voltages V1 and V2 and an additional distinct weakly conductive layer (WCL) 306. The role of the external hole patterned electrode 304 (with voltage V1 applied thereto) is to create a lens-like electric field profile, while the role of the central disk-shaped electrode 305 (with voltage V2 applied thereto) is to avoid disclinations and to control the value of the gradient (e.g., to erase the lens). The role of the WCL 306 is to soften the profile created by V1 and to allow the reduction of the overall thickness of the lens. Unfortunately, the complex patterning of the top electrode, the necessity of using two distinct voltages and a separate WCL are difficult to manufacture and inhibit the practical use of this approach. For example, the use of this approach to build a polarization independent lens would require the use of six to seven thick glasses, which is a difficult task.